How to solve % Abundance problems given average mass and masses of isotopes

In this video, I work through a problem where we're given the average mass of silver, along with the masses of two common isotopes of silver.

Here's a quick overview:

1. write out the average mass as a sum of the isotope, multiplied by its % abundance (x):

average = (isotope1)(x) + (isotope2)(1-x)

(the % abundances MUST add up to 1, which is why we can use only one variable, x, and simply write 1-x for the second isotope.)

2. solve for x

3. put x back into the original equation - if it yields the given average mass, it's correct! If not, something's off


I hope you enjoyed the video - if you have any questions or suggestions for future videos, simply leave a comment below.




Problem Text:
Silver exists in two stable forms: 107Ag (106.90 amu) and 109Ag (108.90 amu). If the average atomic mass of Ag is 107.87 amu, what is the % abundance of 107Ag?

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